Silver-incorporated NiCo metal–organic frameworks with controlled morphology for enhanced cycling in flexible supercapacitor applications

Abstract

Supercapacitors (SCs) are highly promising for next-generation energy-storage applications due to their predominant properties. Metal–organic frameworks (MOFs), an innovative class of energy storage electrodes, offer a wide range of structural variations and high porosities. However, their broad application is limited by their low capacitance and cycle stability. This study uses a facile solvothermal method to synthesize a series of NiCo-MOF (NCA_X-MOF/NF, X = 0, 5, 10, 15, and 20) on nickel foam (NF) with varying amounts of Ag⁺ cations. This study systematically explores the influence of Ag⁺ incorporation on the morphology and electrochemical performance of NiCo-MOF. Analysis of the MOF morphology reveals that the introduction of Ag⁺ transforms the bulk NiCo-MOF into a spherical structure. NCA₁₅-MOF/NF, with its uniform size and spherical morphology, exhibits the best electrochemical performance, achieving a specific capacitance of 1317 F g⁻¹ at 1 A g⁻¹ and retaining 89% of its capacitance after 15 000 cycles. Furthermore, upon assembly into a flexible symmetric supercapacitor, it delivers an energy density of 72.55 W h kg⁻¹ at a power density of 408.61 W kg⁻¹. This study suggests that incorporating Ag⁺ can tailor the MOF morphologies and improve their electrochemical characteristics, thus providing a novel approach for fabricating NiCo-MOF electrodes with enhanced SC performance.

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1. Introduction

The current era is marked by energy-related challenges that are among the most pressing global issues. Concerns pertaining to energy supply and environmental protection require immediate attention. Therefore, it is imperative to devise strategies for efficiently utilizing renewable energy sources while simultaneously promoting the advancement of energy storage and conversion technologies, such as supercapacitors (SCs). SCs are typically classified into three primary classes based on their energy storage mechanisms. The first type is electric double-layer capacitors (EDLCs), which form a Helmholtz layer and a double electric layer at the interface between the electrode and electrolyte. This type of SC capitalizes on its highly specific surface area for energy storage. The second type is pseudocapacitance, in which energy is stored via rapid and reversible chemical adsorption or Faraday redox reactions. The third type is a hybrid supercapacitor, combining the Helmholtz layer and Faraday Redox mechanisms to enhance its energy-storage capabilities.¹ SCs have garnered considerable attention owing to their ability to discharge energy rapidly (i.e., short discharge time, SCs: 1–10 s, and lithium-ion batteries (LIBs): 10–60 min) and enhanced cyclic stability (SCs > 30 000 h and batteries > 500 h).² Integrating SCs with batteries in electric vehicles is a common practice in the electric vehicle industry that aims to extend the lifespan of batteries and address the issue of reduced battery life caused by frequent stops and starts. This approach is particularly relevant in electric vehicles, where using batteries as the sole energy source may result in a decreased battery lifespan.^3,4 SCs are widely valued due to their high specific capacitance, extended operational lifespan, superior power density, and ability to bridge the power and energy gaps between capacitors, fuel cells, and batteries.⁵ These attributes demonstrate the versatile and advantageous nature of SCs in various applications and underscore their significance in pursuing sustainable energy solutions.

Metal–organic frameworks (MOFs) are a class of nanomaterials characterized by their remarkable functional properties and high porosity. Compared to conventional materials, MOFs possess several advantages, such as structural versatility, flexibility, and adaptability, offering abundant pores and porosity (with a BET surface area of approximately 7000 m² g⁻¹ and a pore volume of 4.4 cm³ g⁻¹).⁶ In addition, they have a large surface area and excellent adsorption performance.⁷ The unique characteristics of MOFs, such as their rapid electron transfer and redox properties, make them highly promising materials for various electrochemical applications.^8,9 The unsatisfactory performance of MOFs in high-performance SCs is mainly attributed to their stability issues and low capacitance compared to traditional materials.¹⁰ However, incorporating multiple metals into bimetallic MOFs can lead to a synergistic effect, resulting in enhanced performance compared with monometallic MOFs. The difference in radius between the two metal ions generates a larger number of vacancies and free holes, thereby enhancing the storage capacity of the material.¹¹ Incorporating techniques such as metal/non-metal doping and heterojunction engineering provides a highly effective approach for improving the electrochemical performance of materials. Electrochemical properties, such as improved cycle stability, can be achieved by optimizing the material structure and minimizing internal resistance. Research has shown that larger-radius metal ions can be successfully doped into energy storage materials. Liu et al.¹² successfully synthesized Se–Co-MOFs exhibiting enhanced electrochemical reactivity, conductivity, and stability by incorporating Se into Co-MOFs. Even after 10 000 cycles, the mass-specific capacitance maintained a notable retention rate of 70%. Cao et al.¹³ dispersed Ag nanoparticles, approximately 20 nm in size, incorporated between the Ni-MOF nanosheets. This integration increased the surface area and pore size of the Ni-MOF, leading to a significant increase in the specific capacitance from 820 F g⁻¹ (Ni-MOF) to 1312 F g⁻¹. Wang et al.¹⁴ used a polyoxometalate-based MOF (POMOF) doped with Ag⁺ as the electrode material for both SCs and catalysts. They achieved a specific capacitance of 533.6 F g⁻¹ at a current density of 1 A g⁻¹. The introduction of Ag⁺ doping enhanced the conductivity, capacitance, and surface area of the materials, consequently augmenting the specific capacitance of the supercapacitors. Similarly, this doping induces alterations in the electronic structure, improving stability.¹⁵

In the realm of MOF electrodes, using porous flexible materials as self-growing substrates is a promising alternative to incorporating binders into conventional electrode materials.^16–18 This incorporation of binders tends to decrease the available surface area of the electrode and impede charge and ion transport between the electrode and electrolyte, hindering the overall electrochemical energy storage performance.¹⁹ Integrating knowledge from these two interconnected spheres highlights the significance of innovating MOF electrodes that are not only structurally flexible but also free from the performance-compromising effects of binders, thus setting the stage for the advancement of next-generation energy storage devices. Concurrently, the growing demand for flexible devices driven by the rapid development of portable power devices has made flexible quasi-solid-state SCs an important area of contemporary and prospective research.^20,21 This trend aligns with the need for energy storage solutions that can be seamlessly integrated into flexible electronic devices.

This study addressed the issues associated with particle agglomeration, volume expansion, collapse susceptibility, and other defects in bimetallic NiCo-MOFs. This study adopted a self-growth method using Ni(NO₃)₂·6H₂O and Co(NO₃)₂·6H₂O as the starting materials and three-dimensional (3D) flexible nickel foam (NF) as the substrate. The incorporation of Ag (as zero-valent) was introduced to improve the internal structure of the electrode, optimize the charge distribution, and enhance the conductivity of the NiCo-MOF bimetallic material. Ag particles, due to their unique properties (i.e., ion size and geometry, electronegativity and polarizability, variety of oxidation states, and catalytic properties), function as structure-directing agents during the MOF synthesis process. Ag particles can significantly influence the growth kinetics of the MOF crystals, leading to an alteration in the morphology. In particular, Ag particles promote localized charge distribution, which influences the initial nucleation processes and facilitates the formation of smaller crystal domains. This effect transforms the bulk NiCo-MOF structure into a more uniform spherical morphology. These morphological changes significantly enhance ion transport and diffusion during electrochemical processes, increasing the electrochemically active surface area and, furthermore, the capacitance of the material. Furthermore, Ag particles have a lower reduction potential than Ni²⁺ and Co²⁺ ions, allowing them to readily participate in redox reactions during the charge/discharge cycles of the SCs. The strategic incorporation of Ag changes the local electronic environment around the active sites, facilitating more efficient redox processes and improved electrochemical performance of the NiCo-MOF electrode. Similarly, introducing Ag leads to surface roughening of the NiCo-MOF and reduces particle size. Moreover, the presence of numerous pores within the spherical structure increased the specific surface area, facilitating charge transfer and resulting in superior electrochemical performance.

A device was assembled to investigate the practical applications of this material. A gel electrolyte, which is necessary to prepare flexible quasi-solid symmetric SCs, was incorporated between the negative and positive electrodes. This addition increased the pseudocapacitance and working potential of the device. The unobstructed ion shuttle and efficient charge transfer between the two electrodes significantly contributed to the enhanced performance of the SC device. Additionally, the excellent ionic conductivity of high-concentration KOH and the excellent stability of the PVA chains could enable a wider operating potential window for the device.

2. Experimental section

2.1 Materials

Analytical grade reagents and chemicals used in this study, including 1,3,5-benzene tricarboxylic acid (H₃BTC), nickel nitrate hexahydrate (Ni(NO₃)₂·6H₂O), cobalt nitrate hexahydrate (Co(NO₃)₂·6H₂O), potassium hydroxide (KOH), N,N-dimethylformamide (DMF), anhydrous ethanol (C₂H₅OH), silver nitrate (AgNO₃), hydrochloric acid (HCl), and polyvinyl alcohol (PVA), were purchased from Sinopharm (Group Chemical Reagent Co., Ltd, China). NF was used as the scaffold electrode substrate (Jiangsu Kunshan Billion New Materials Co. Ltd, China).

2.2 Preparation of the NCA_X-MOF/NF electrode

The NF substrate was cut into the desired dimensions of 1.5 cm × 3 cm and underwent a surface pretreatment procedure. A pretreatment step was performed by sequentially immersing the NF in 5% hydrochloric acid (HCl), anhydrous ethanol, and deionized water, followed by ultrasonic treatment at room temperature for 10 min. The pretreated NF were then exposed to a vacuum oven at 60 °C for 24 h. Next, five solutions were prepared by dissolving 1.5 mmol H₃BTC in DMF (60 mL). The solution was stirred at a constant rate for 30 min at room temperature. Subsequently, 3.0 mmol Ni(NO₃)₂·6H₂O and 1.5 mmol Co(NO₃)₂·6H₂O powders were added to each of the five solutions. After thorough stirring, different volumes (0, 5, 10, 15, and 20 mL) of 1.0 M AgNO₃ were added to the solution. The resulting solution was stirred in the dark at room temperature. Next, the mixture solutions were transferred into 100 mL polytetrafluoroethylene (PTFE) liners, and the pretreated NF was placed inside. The PTFE liners were tightly sealed in a stainless-steel autoclave, and the reaction was conducted for 15 h at 120 °C in a hydrothermal reactor. Upon completion of the reaction, the samples were washed thrice with DMF, deionized water, and anhydrous ethanol. After washing, the samples were dried in a vacuum oven at 60 °C for 24 h. The electrode materials synthesized through this process were denoted as NCA₀-MOF/NF, NCA₅-MOF/NF, NCA₁₀-MOF/NF, NCA₁₅-MOF/NF, and NCA₂₀-MOF/NF, representing the different compositions of the MOF on the NF. A schematic of the experimental process is shown in Fig. S1 (ESI†).

2.3 Characterization of materials

X-ray diffraction (XRD), Raman spectroscopy (Raman), and X-ray photoelectron spectroscopy (XPS) were used to examine the crystallographic properties and elemental composition of the samples. Scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM) were used to visualize the surface morphologies, spatial structures, and crystallography of the materials, and Brunauer–Emmett–Teller (BET) analysis was performed to explore the specific surface areas and pore structures of the samples. Inductively coupled plasma optical emission spectroscopy (ICP-OES) was used to detect the content of Ag in materials. The relevant instruments and characterization parameters are provided in the ESI.†

2.4 Electrochemical measurements

The electrochemical properties of the material were assessed using cyclic voltammetry (CV), galvanostatic charge–discharge (GCD), and electrochemical impedance spectroscopy (EIS) measurements in a three-electrode system using a CHI electrochemical workstation (CHI760E, Chen Hua Instrument Co. Ltd, Shanghai, China). The synthesized material grown on NF was used as the working electrode (WE), a platinum sheet served as the counter electrode (CE), and a Hg/HgO electrode functioned as the reference electrode (RE). A 3.0 M potassium hydroxide (KOH) solution was used as the electrolyte for electrochemical measurements.

2.5 Assembly and electrochemical measurements of flexible quasi-solid-state symmetric SCs

The electrode with the most favorable electrochemical performance was used as both the positive and negative electrodes for SCs. A gel electrolyte composed of PVA/KOH was prepared by carefully mixing 3.0 g of PVA, 1.68 g of KOH, and 30 mL of deionized water, then heating the mixture to 85 °C in a water bath. Next, the positive electrode, gel electrolyte, and negative electrode were integrated to fabricate flexible quasi-solid-state symmetric SCs. The synthesized positive and negative electrodes were immersed in a gel electrolyte for 8 min. The electrodes were removed from the gel electrolyte and assembled in the correct order and orientation. The assembled components were allowed to cool and solidify at room temperature, and the resulting solid structure was encapsulated to form flexible symmetric SCs.

3. Results and discussion

3.1 Morphology and composition

The crystal structures of NCA_{(0,5,10,15,20)}-MOF/NF were obtained using XRD and Raman analyses, as shown in Fig. 1. As shown in Fig. 1a, the crystal structures of the NCA_{(0,5,10,15,20)}-MOF/NF patterns were compared with the standard reference (PDF#65-2871) from the ICDD database. The XRD results showed the prominent diffraction peaks of Ag at approximately 38.1°, 44.3°, 64.5°, and 77.0°, which correspond to the (111), (200), (220), and (311) crystal planes of elemental Ag (as a zero-valent), respectively (Fig. 1a). These results confirmed the presence of Ag as elemental silver within the composite structure (Fig. 1a). To further elucidate the presence of NiCo-MOF, XRD testing was conducted at low angles, resulting in the data exhibited in Fig. 1b, which indicate the presence of NiCo-MOF. The findings showed that the diffraction peaks of the prepared NCA_{(0,5,10,15,20)}-MOF/NF are consistent with those of Ni-BTC (CCDC: No. 2128682) and Co-BTC (CCDC: No. 2128895), indicating the successful synthesis of NiCo-MOF. The peaks at 17.56°, 18.77°, 20.12°, and 22.06° correspond to the (220), (111), (221̄), and (400) crystal planes of Ni-BTC, respectively (Fig. 1b).

Fig. 1 Structural characterization of NCA_X-MOF/NF samples: XRD patterns of (a) the distinct peaks of the elemental Ag incorporated NiCo-MOF structure and (b) NiCo-MOF at low angles. (c) Raman spectra show key peaks for Ni-BTC and Co-BTC coordination complexes, confirming the MOF structure. (The absence of shifts or new peaks in the NCA_X-MOF/NF spectrum indicated that Ag was in its elemental form (Ag⁰) and not coordinated with the MOF ligands or metal centers.)

According to the Raman spectra shown in Fig. 1c, characteristic peaks of the NiCo-MOF are clearly observed, confirming the integrity of the MOF structure. The peak at 1204 cm⁻¹ corresponds to the bending vibration of the C–H plane in the benzene ring, a feature typically associated with the organic ligands of the MOF. The peak observed at approximately 807 cm⁻¹ is attributed to the asymmetric stretching mode of oxygen within the O–M–O bond, indicative of the metal–oxygen coordination within the MOF framework.²² Additionally, the peak at 1044 cm⁻¹ represents the C=C vibration of the aromatic ring, while the peak at 1476 cm⁻¹ is associated with COO– groups, confirming the presence of carboxylate functionalities.²³ The peak at 3036 cm⁻¹ is assigned to the stretching vibration of the C–H plane in the benzene ring.

Strikingly, no significant shifts in the Raman characteristic peaks of the MOF or the emergence of new vibrational modes were detected in the Ag-incorporated NCA_X-MOF/NF samples compared to the pristine NiCo-MOF/NF. This suggests that the Ag incorporation does not result in coordination with MOF ligands or metal centers, indicating that Ag remains in its elemental form (Ag⁰). Moreover, the absence of peaks typically associated with silver oxides (e.g., Ag₂O or AgO around 500–700 cm⁻¹) further confirms that Ag was not oxidized and remained in its elemental state without forming coordination bonds with the MOF structure.

XPS analysis of NCA_X-MOF/NF, following irradiation with an Al-Kα source, was performed to investigate the valence states of its constituent elements, as depicted in Fig. 2. The spectrum shown in Fig. 2a revealed the presence of Ni, Co, O, C, and Ag in the NCA_X-MOF/NF. Fig. 2b shows the fine spectrum of the C 1s region, with the dominant peak observed at 284.78 eV corresponding to the C=C bond vibration in H₃BTC. The weaker peaks at 286.38 and 288.58 eV could also be attributed to the C–O and –O–C=O bond vibrations in H₃BTC, respectively. Further analysis of the fine spectrum of the O 1s region (Fig. 2c) revealed a peak at 531.78 eV, indicating the presence of the M–O bond in NCA_X-MOF/NF, while a weak peak at 533.18 eV was attributed to the vibration of the C–O bond. In the narrow spectrum of the Ni 2p region (Fig. 2d) of NCA₁₅-MOF/NF, peaks corresponding to the 1/2 and 3/2 orbitals of Ni 2p were observed at about 874.68 eV and 856.86 eV, respectively. Interestingly, NCA₁₅-MOF/NF exhibited a higher Ni 2p binding energy than the other samples, indicating a lack of electronic states. This phenomenon could be attributed to enhancing the internal electron binding energy resulting from oxidation, where greater electron loss led to higher oxidation states.²⁴ Similarly, in the fitted narrow spectrum of the Co 2p region shown in Fig. 2e, the peaks at 797.78 eV and 781.48 eV corresponded to the 1/2 and 3/2 orbitals of the Co 2p, respectively. The presence of weak peaks at 802.78 eV and 786.78 eV in the narrow spectrum indicated the presence of satellite peaks, confirming the presence of Co²⁺ in the material. Finally, in the fitted narrow spectrum of the Ag 3d (Fig. 2f), peaks at 374.48 eV and 368.68 eV corresponded to the 3/2 and 5/2 orbitals of Ag 3d, respectively, confirming the presence of Ag.^25,26

Fig. 2 XPS characterization of NCA_X-MOF/NF: (a) Full spectrum, (b) C 1s spectrum, (c) O 1s spectrum, (d) Ni 2p spectrum, (e) Co 2p spectrum, and (f) Ag 3d spectrum.

The effects of Ag incorporation on the morphology of the materials were visually observed using SEM images. Fig. 3a–f show the SEM images of NF, NCA₀-MOF/NF, and NCA₁₅-MOF/NF at different magnifications, revealing distinct differences in the micromorphology. The surface of the pure NF exhibited a relatively smooth morphology (Fig. 3a and d).²⁷ In contrast, the introduction of NiCo-MOF to the NF surface led to the formation of block-like structures through layer stacking, with an average diameter of 40 μm (Fig. 3b and e). However, these block-like particles underwent a transformative process upon introducing Ag ions, converting into nanospheres with a significantly uniform diameter of 2 μm, as shown in Fig. 3c and f. Strikingly, this diameter was significantly smaller than the block-like particles observed in the NCA₀-MOF/NF. The presence of the block-like NiCo-MOF impaired the electron and ion transport pathways owing to their stacking arrangement. However, the spatial structure of NCA₁₅-MOF/NF exhibited distinct advantages, as inferred from its morphology, which was expected to enhance its electrochemical performance. These findings suggest that the addition of Ag particles has a significant impact on the morphology of the NiCo-MOF growth on the NF sheet. To further explore the influence of Ag particle incorporation on the materials, three different MOF materials, NCA_(5,10,20)-MOF/NF, were characterized by SEM analysis (Fig. S2, ESI†). As shown in Fig. S2a–d (ESI†), it is evident that regardless of the Ag particles incorporating concentration (5 mL or 10 mL), the loading of the active material on the NF was insufficient, resulting in an uneven morphology and size of the NiCo-MOF. This lack of uniformity may be attributed to inadequate binding between the MOF and the NF during the reaction. Nevertheless, when the Ag particles incorporating concentration reached 15 mL, as shown in Fig. 3c and f, there was a significant increase in the quantity of NCA₁₅-MOF particles loaded onto the NF. This increase was accompanied by a more uniform dispersion and particle size. These findings indicated that the Ag particles incorporating concentration significantly impacts the quantity and morphology of NiCo-MOF grown on NF. This finding suggested that a higher concentration of Ag enhanced the loading of NCA₁₅-MOF particles on the NF, resulting in a more uniform dispersion and structure. This development was attributed to the increased incorporation of ions, which increased the surface potential barrier and reduced the width of the space-charge region. Thus, the electron–hole pairs within this region experienced effective separation facilitated by a significant electric field, mitigating the possibility of prompt recombination.²⁸ However, at a higher incorporation amount of Ag, 20 mL, the morphology of the NiCo-MOF underwent significant changes. Excessive rod-shaped structures were interconnected to form a network-like structure on the NF substrate (Fig. S2e and f, ESI†). This phenomenon could be due to the presence of weak van der Waals forces,²⁹ which enhanced the adherence of the NiCo-MOF particles, leading to the formation of micrometer-sized rod-shaped morphologies. TEM and EDS characterizations were performed to further investigate the microstructure of NCA₁₅-MOF/NF, revealing that the Ag was incorporated with NiCo-MOF. The observed lattice spacings of 0.2077 nm and 0.2334 nm corresponded to the (133) crystal plane of NiCo-MOF and the (111) crystal plane of Ag, respectively (Fig. 3g–n). The Ag concentration incorporated into each NCA_X-MOF/NF electrode was determined using ICP-OES analysis, as shown in Table S1 (ESI†). The measured Ag concentration in the NCA₁₅-MOF/NF sample was approximately 38.90 mg g⁻¹ (Table S1, ESI†).

Fig. 3 SEM images of NF, NCA₀-MOF/NF, and NCA₁₅-MOF/NF: (a) and (b) NF, (c) and (d) NCA₀-MOF/NF, and (e) and (f) NCA₁₅-MOF/NF. NCA₁₅-MOF/NF of the corresponding elemental mapping images (g)–(l) and HRTEM of NCA₁₅-MOF/NF (m) and (n).

Fig. S3 (ESI†) shows the N₂ adsorption–desorption isotherms, pore size distribution, specific surface area, and pore volume of NCA_X-MOF/NF. Strikingly, all observed adsorption–desorption isotherms entirely confirmed H3 hysteresis loops (type IV). The incorporation of Ag into the composite material led to a substantial increase in the specific surface area compared with that of NCA₀-MOF/NF, with NCA₁₅-MOF/NF displaying the largest specific surface area. This augmentation not only facilitated the efficient utilization of active materials but also accelerated ion transfer, thereby optimizing the electrochemical performance. Furthermore, due to the addition of Ag⁺, compared to NCA₀-MOF/NF (11.47 nm), the pore sizes of NCA_X-MOF/NF (X = 5, 10, 15, 20) were 14.33 nm, 16.08 nm, 16.09 nm, and 12.44 nm, respectively. The presence of mesopores and macropores contributes to the enhanced ion transport within the electrolyte.³⁰

3.2 Electrochemical measurement of the materials

The synthesized NCA_X-MOF/NF exhibited a favorable electron/ion transport structure, prompting further investigation of its capacitance. To compare the electrochemical performances of NCA₀-MOF/NF, NCA₅-MOF/NF, NCA₁₀-MOF/NF, NCA₁₅-MOF/NF, and NCA₂₀-MOF/NF as SC electrodes, a three-electrode system was employed to evaluate their electrochemical performance in a 3.0 M KOH electrolyte. Before the analysis, the unit mass loads of these materials were calculated as 1.68, 1.74, 1.71, 1.63 and 1.76 mg cm⁻², respectively.

Fig. 4a shows the CV curves of the four NCA_X-MOF/NF composites at a scanning rate of 100 mV s⁻¹. The CV curves are characterized by a quasi-rectangular shape with complex redox behavior, indicating faradaic reactions associated with various active sites. This Faraday characteristic arises from the surface redox reactions of Ni²⁺/Ni³⁺ and Co²⁺/Co³⁺ and the contribution from Ag⁰ redox reactions occurring during electrochemical measurements. Thus, the charge storage mechanism can be described by the following redox reactions near 0.3 V, which can be attributed to the reactions as indicated in eqn (1), (2) and (4).³¹ Notably, the anodic peaks appeared broadened and less defined, with a more pronounced anodic peak at a lower scan rate (Fig. S4, ESI†). The broad nature and poorly defined anodic peaks suggest that the oxidation process involves multiple intermediate steps, which distribute the oxidation current across a wide potential range and result in less sharp peaks. This phenomenon may be attributed to slower electron exchange rates or diffusion limitations, particularly in systems where ion diffusion through MOF pores is significant.³² Furthermore, surface restructuring may hinder the oxidation process, leading to less distinct anodic peaks. In the incorporation of Ag, the anodic peak in the range of 0.2 V is associated with the formation of the Ag₂O layer. The cathodic peak around 0.1 V describes the reduction of Ag₂O back to Ag⁰, as described in eqn (3).³³ Furthermore, the integrated area of the CV curve for NCA₁₅-MOF/NF was significantly larger than that of the other four composites. This indicates that NCA₁₅-MOF/NF exhibits the highest charge storage capacity among the composites, suggesting its superior electrochemical performance in capacitance. This enhancement can be attributed to the excellent electrical conductivity of the Ag particles, which served as conductive bridges that facilitate rapid electron transfer during redox reactions.^34,35 Similarly, Co ions, particularly Co²⁺, participate in redox reactions (Co²⁺/Co³⁺), enhancing the faradaic charge storage and specific capacitance of the electrode,³⁶ as described in eqn (4). This additional redox activity enhances the overall faradaic charge storage of the material, contributing to the specific capacitance of the electrode. The introduction of Co³⁺ ions facilitates the mitigation of the Jahn–Teller distortion that typically occurs with Ni³⁺ ions.³⁷ This structural stabilization during electrochemical cycling improves the cycling stability and durability of the electrode material, leading to better performance over extended charge/discharge cycles. In our previous study, we used density functional theory (DFT) analysis to examine the electrochemical effects of Co²⁺ in Ni-based MOFs.⁸ The results indicated that incorporating Co²⁺ leads to strong orbital hybridization between Ni 3d, Co 3d, C 2p, O 2p, and N 2p states near the Fermi level.⁸ This hybridization enhances the electron transfer capability and significantly improves the kinetics of redox reactions, resulting in accelerated redox reaction rates. Moreover, the introduction of Co²⁺ promotes the adsorption of OH⁻ on Ni-MOF, and the reduction in adsorption energy further promotes the redox reaction between electrolyte ions and electrode materials.⁸

Ni(II) + OH⁻ ↔ Ni(II)OH + e⁻ (1)

Ni(II)OH ↔ Ni(III)(OH) + e⁻ (2)

2Ag + 2OH⁻ ↔ Ag₂O + H₂O + 2e⁻ (3)

Co(II) + OH⁻ ↔ Co(III)(OH) + e⁻ (4)

Fig. 4 Electrochemical performance of NCA_X-MOF/NF electrodes: (a) CV, (b) GCD, (c) EIS, and (d) Coulombic efficiency and specific capacitance curves.

Fig. 4b shows the GCD curves of the different composites at a current density of 1 A g⁻¹. A distinct step can be observed in the nonlinear region of the curves, suggesting that the energy behavior of the electrodes is primarily governed by ion diffusion and redox processes. Strikingly, the GCD curves of the electrodes exhibited a unique asymmetric triangular shape, particularly in the middle and late stages of the discharge process, under the influence of different Ag⁺ incorporating amounts. The discharge platform became more pronounced with increasing Ag⁺ concentration. The GCD curve of NCA₂₀-MOF/NF exhibited a distinct ohmic drop, likely due to the modified shape of the NiCo-MOF loaded on NF. This modification led to suboptimal contact with the electrolyte, increasing the resistance and causing the observed ohmic drop phenomenon.³⁸ The inconsistent potential platform during discharge can be attributed to the dual effect of Ag particles with the porous characteristics of the NiCo-MOF material. The change in the porous structure may reduce the number of active sites and limit the mass diffusion to these sites, resulting in uneven electrochemical reactions.³³ In addition, Ag particles form nanoscale conductive contacts during the charge and discharge process, promoting rapid electron transfer and providing additional redox capacity. As the Ag₂O–Ag core–shell structure forms and transforms, the potential barrier created by the thick Ag₂O layer prevents further oxidation of the Ag spheres. Differences in the content or uneven distribution of added Ag lead to varying potential barriers generated by the thick Ag₂O layer, resulting in multiple discharge voltages.³⁹

To further investigate the electrochemical performance of the four composites, EIS measurements were performed under open-circuit voltage (OCV) conditions within the frequency range of 10⁻² to 10⁴ Hz. Fig. 4c shows the fitted EIS results and the corresponding equivalent circuit model of the electrode material. A detailed illustration is provided in the inset of Fig. 4c. In the EIS curve, the intersection point of the semicircular arc with the x-axis represents the intrinsic resistance of the material, which is denoted as the equivalent series resistance R₁. The radius of the semicircle represents the charge transfer resistance, which is referred to as R₂. Furthermore, the inverse of the slope of the linear segment of the curve, represented as W_O, corresponds to the resistance associated with ion transport and diffusion. A steeper slope indicated a lower charge-transfer resistance (R_CT), demonstrating superior conductivity. The determined R₁ of approximately 0.92 Ω for the five composites suggests a consistent value. However, variations were primarily observed in R₂ and W_O. Further analysis of the EIS diagram revealed that the NCA₁₅-MOF/NF composite exhibited the steepest slope in the linear curve region, which corresponds to the low-frequency region. This finding suggests a smaller impedance for ion transmission and diffusion in NCA₁₅-MOF/NF. In contrast, in the high-frequency region, the radius of the semicircular arc associated with NCA₁₅-MOF/NF was significantly larger than those of NCA₅-MOF/NF, NCA₁₀-MOF/NF, and NCA₂₀-MOF/NF. This finding reveals that NCA₁₅-MOF/NF possesses a lower resistance to charge transfer, while NCA₅-MOF/NF, NCA₁₀-MOF/NF, and NCA₂₀-MOF/NF displayed relatively higher R_CT. The results demonstrated that transition metal incorporation enables faster faradaic charge transfer and ion diffusion processes at the electrode/electrolyte interface. Simultaneously, this phenomenon may be attributed to the fact that NCA₁₅-MOF/NF possesses the highest specific surface area, promoting efficient electron transfer and facilitating the insertion and extraction of electrolyte ions.^40,41

Based on the GCD curve (eqn (S1), ESI†) corresponding to the electrode, the specific capacitance value of the electrode could be determined. As shown in Fig. 4d, the specific capacitances of NCA₀-MOF/NF, NCA₅-MOF/NF, NCA₁₀-MOF/NF, NCA₁₅-MOF/NF, and NCA₂₀-MOF/NF at a current density of 1 A g⁻¹ were approximately 310, 705, 820, 1317, and 630 F g⁻¹, respectively. The Coulomb efficiencies of the corresponding electrode configurations, obtained using eqn (S2) (ESI†), were 96.8%, 69.5%, 67.6%, 89.9%, and 84%, respectively. The Coulombic efficiencies of NCA₅-MOF/NF and NCA₁₀-MOF/NF were notably lower than those of NCA₀-MOF/NF and NCA₁₅-MOF/NF, respectively. This observation may be attributed to the inferior crystallinity of the MOF and the associated larger R₂, which hindered effective electron transfer and consequently resulted in reduced Coulombic efficiency.⁴¹ Although the Coulombic efficiency of NCA₁₅-MOF/NF was slightly lower than that of NCA₀-MOF/NF, its electrochemical performance was significantly improved, which is crucial for the practical utilization of the device.

As shown in Fig. 4a, the incorporation of Ag distinctly increased the specific capacitance of the material. The CV curves of the synthesized anode were measured at varying scan rates of 10 to 100 mV s⁻¹ within a 3.0 mol L⁻¹ KOH electrolyte, within a potential range from −0.2 to 0.4 V (Fig. S5, ESI†). These curves were utilized to calculate the total voltammetric charge (q_T*) (eqn (S3)–(S5), ESI†), which directly indicated the number of active sites on the electrode surface.⁴² Fig. S6 (ESI†) shows the linear relationships of (q*)⁻¹versus v^1/2 and q* against v^−1/2, with the results detailed in Table S2 (ESI†). The quantified total voltammetric charges for NCA₀-MOF/NF, NCA₅-MOF/NF, NCA₁₀-MOF/NF, NCA₁₅-MOF/NF, and NCA₂₀-MOF/NF were determined to be 0.514 mC cm⁻², 1.014 mC cm⁻², 11.411 mC cm⁻², 12.710 mC cm⁻², and 2.414 mC cm⁻², respectively. These findings indicated that the incorporation of Ag significantly enhances the number of active sites available on the electrode material. Notably, the inner voltammetric charge (q_I) for the NCA₁₅-MOF/NF electrode was measured at 12.494 mC cm⁻², approximately 30.17 times higher than that of the NCA₀-MOF/NF electrode (0.414 mC cm⁻²), demonstrating superior porosity in the NCA₁₅-MOF/NF structure. This enhanced porosity is beneficial to electrolyte infiltration into the internal active sites, significantly enhancing the overall performance of the electrode material. In conjunction with the Brunauer–Emmett–Teller (BET) analysis, the incorporation of Ag significantly increased the surface area of the electrode and facilitated the development of pore size. These enhancements improved electrolyte permeability and ion transport efficiency and optimized the electrochemical performance of the electrodes. Such improvements enable more effective utilization of active materials, increase the exposure of active sites, and enhance both charge storage and transport capabilities, ultimately leading to superior electrochemical performance.

Based on the electrochemical analysis, NCA₁₅-MOF/NF exhibited superior electrochemical performance, necessitating a thorough investigation of its energy-storage mechanism. CV measurements at varying scanning rates and GCD measurements at different current densities were performed on NCA₁₅-MOF/NF. As shown in Fig. 5a, the CV curves retained similar profiles at different scan rates. Distinct oxidation–reduction peaks were observed at a scanning rate of 10 mV s⁻¹. However, with increasing scanning rate, the oxidation and reduction peak potentials shifted positively and negatively, respectively, owing to the increased ohmic resistance and polarization effects. As the scanning rate increased, the oxidation peak, originally at 0.27 V, shifted to 0.3 V and became obscured, resulting in diminished visibility of the oxidation peak in Fig. 5a. This phenomenon is attributed to the combined effects of increased ohmic resistance and polarization. The GCD analysis and eqn (S6) (ESI†) calculations, as shown in Fig. 5b, demonstrated that approximately 56% of the total capacitance at a scan rate of 5 mV s⁻¹ in NCA₁₅-MOF/NF was attributable to the capacitance resulting from the surface-controlled process. This finding suggests that the electrochemical behavior of NCA₁₅-MOF/NF is predominantly governed by faradaic redox reactions. A subsequent increase in the scan rate amplified the intensity of the faradaic redox reaction and ion diffusion. Thus, increasing the scanning rate from 5 mV s⁻¹ to 100 mV s⁻¹ led to a gradual increase in the proportion of surface-controlled capacitors from 56% to 77%.

Fig. 5 Electrochemical characterization of the NCA₁₅-MOF/NF energy storage mechanism: (a) CV curves at different scan rates, (b) capacitance contributions from the surface and diffusion-controlled processes at various scan rates, (c) GCD curves at different current densities, (d) specific capacitance values at varying current densities, and (e) Nyquist plot from EIS analysis.

As shown in Fig. 5c and d, the mass-specific capacitances of NCA₁₅-MOF/NF at different current densities were 1626, 1317, 990, 891, and 640 F g⁻¹ for current densities of 0.5, 1, 2, 5, and 10 A g⁻¹, respectively, confirming excellent electrochemical properties of the material. The peak observed in the GCD curve can be attributed to the presence of a significant number of pores within the MOF structure during electrochemical reactions. These pores served as reservoirs and accumulated electrolyte ions. Upon reaching a specific potential, the accumulated ions participate in redox reactions with the active material.^43–45 As earlier indicated, the incorporation of Ag⁺ within the NiCo-MOF lattice led to an additional redox reaction resulting from the alteration of the oxidation states of the Ni ions in the electrolyte. At relatively low current densities, two distinct turning points were observed in the GCD. This is due to the oxidation–reduction reaction of Ni(II) and Ag⁰, and the depression was caused by the oxidation–reduction reaction on the electrode surface, which led to the depletion of ions in the electrolyte.⁴⁶ However, the ions in the solution near the electrode material were not replenished, resulting in a certain concentration difference, leading to concentration polarization and a sharp decrease in the curve. When ions in the electrolyte diffuse to the surface of the electrode material, the curve rises again, forming a depression.^47,48 Under a high current density, the concentration polarization arising from the constrained ion transport rate within the electrolyte may undergo considerable augmentation. This polarization causes the ions in the electrolyte to take a longer path to reach the electrode surface, thereby reducing the response speed of the SCs. This may have led to a decrease in the plateau of the GCD curve. This reverse alteration in the oxidation state generates a voltage output phenomenon. As shown in Fig. 5e, the low R_S and ion/electron transfer resistance of NCA₁₅-MOF/NF further indicated its superior electrochemical performance.

The XRD patterns for NCA₁₅-MOF were analyzed before and after cycling tests, as shown in Fig. 6a. Before the cycling tests, the XRD pattern of NCA₁₅-MOF exhibited sharp and well-defined diffraction peaks that corresponded to the characteristic angles of Ni-BTC (CCDC: No. 2128682) and Co-BTC (CCDC: No. 2128895). This observation indicated the stable framework of the material, which is favorable to charge storage. Following the cycling test, a significant alteration in the XRD pattern was observed (Fig. 6a). The peaks associated with the original NiCo-MOF framework exhibited a significant reduction in intensity, indicating a degradation of the crystalline structure. This transformation suggested a phase transition from the original MOF to the formation of layered double hydroxides (LDHs).⁴⁹ This structural change can be ascribed to the electrochemical reactions occurring within the material. Specifically, during the charge/discharge cycles, the interaction between hydroxide ions (OH⁻) and Ni²⁺ ions promotes the conversion of Ni²⁺ into nickel hydroxide (Ni(OH)₂) under alkaline conditions (eqn (5) and (6)). This process may dominate any reactions involving Co²⁺, leading to the formation of cobalt hydroxides, particularly nickel–cobalt layered double hydroxides (NiCo-LDHs), which serve as precursors in this study.

Ni(II) + 2OH⁻ → Ni(OH)₂ (5)

Ni(OH)₂ + Co²⁺ + 2OH⁻ → NiCo-LDH (6)

Fig. 6 Characterization of NCA₁₅-MOF/NF before and after the cycle tests: (a) XRD patterns, (b) EIS plots, and (c) specific capacitance–Coulomb efficiency.

The gradual disappearance of the peaks corresponding to the NCA₁₅-MOF suggests that the original framework is breaking down, while new peaks corresponding to the NiCo-LDH structure may begin to emerge. However, such peaks might not be prominently distinct due to the amorphous nature of the resultant material.

As shown in Fig. 6a, there were changes in the XRD pattern of NCA₁₅-MOF/NF after cycling. This may be attributed to the reaction between OH⁻ and Ni²⁺, as indicated in eqn (1) and (2), leading to a gradual transformation of the electrode material into NiCo-LDH after cycling, while the original NiCo-MOF peak diminishes progressively. As shown in Fig. 6b, the EIS analysis before and after cycling revealed a slight increase in R_CT, while the R_S remained relatively unchanged. This suggests that although there is a minor rise in resistance due to structural changes in the electrode material, likely caused by the interaction with OH⁻ ions from the electrolyte, and the overall impact on ion and electron transport is minimal.^50–54 Furthermore, the formation of NiCo-LDH during cycling stabilizes the material's structure (Fig. S7c and d, ESI†), which facilitates better cycling stability despite minor resistance changes. Therefore, enhanced cycling stability is ensured through the synergistic effects of Ag incorporation and the structural transformation that occurs during the cycling process. Similarly, the surface of the NCA₁₅-MOF/NF before cycling exhibited a distinct outline and abundant pores. However, after cycling, an additional layer of crystalline material was observed on the electrode surface, which interfered with the subsequent interaction between the electrolyte ions and the active materials of the electrode (Fig. S7c and d, ESI†). As a result, the capacitance of NCA₁₅-MOF/NF underwent a certain decrease after cycling compared with its initial state. Furthermore, the HRTEM image after cycling showed that NCA₁₅-MOF/NF underwent significant changes compared to the result obtained in Fig. 3g, with an increased presence of flake-like structures. This finding indicated the transformation from NiCo-MOF to NiCo-LDH (Fig. S7e, ESI†). Similarly, the lattice spacing of NiCo-LDH surrounding the Ag particles is measured at 0.2353 nm (Fig. S7f, ESI†). These results, in conjunction with the XRD analysis obtained after cycling, correspond to the (015) crystal plane of NiCo-LDH (Fig. S7g, ESI†).

To assess the cycling stability of the NCA₁₅-MOF/NF material, the electrode was subjected to continuous charge/discharge tests over 15 000 cycles within a potential window of −0.2 to 0.4 V at a current density of 5 A g⁻¹, using a platinum plate as the counter electrode (CE) (Fig. 6c). As shown in Fig. 6c, the NCA₁₅-MOF/NF exhibited superior performance, with the specific capacitance decreasing gradually with increasing cycle number, maintaining 89% of its initial specific capacitance and 90% Coulombic efficiency after 15 000 cycles (Fig. 6c). In comparison, the Ag-free NCA₀-MOF/NF electrode was tested under the same conditions as NCA₁₅-MOF/NF for 6000 cycles (Fig. S8, ESI†). The results indicated a significant drop in performance, with specific capacitance and Coulombic efficiency decreasing to 64.68% and 70.8%, respectively (Fig. S8, ESI†). These findings demonstrated that the NCA₁₅-MOF/NF material, with Ag incorporation, offers significantly enhanced electrochemical stability over long-term cycling compared to the NCA₀-MOF/NF material. Similarly, as shown in Fig. S7e (ESI†), the HRTEM image of NCA₁₅-MOF/NF after cycling revealed significant changes compared to the pre-cycling image in Fig. 3(g), with increased flake-like structures. This transformation suggests the conversion of NiCo-MOF to NiCo-LDH. Furthermore, the lattice spacing of NiCo-LDH surrounding the Ag particles is measured at 0.2353 nm, corresponding to the (015) crystal plane of NiCo-LDH, as confirmed by the XRD analysis conducted after cycling (Fig. S7g, ESI†).

As depicted in Fig. 7, Ag acts as a nucleation site, promoting the growth of NiCo-MOF crystals around its surface. The presence of Ag facilitates the dispersion and formation of smaller MOF particles, thus increasing the overall surface area. This increased surface area enhances ion diffusion and provides more active sites for redox reactions during the electrochemical process. Furthermore, Ag particles contribute to improved electron conductivity within the structure. Ag with high conductivity creates efficient pathways for electron transfer, reducing internal resistance and enhancing charge transfer between the active material and the electrode substrate. The formation of this interconnected network of MOF particles surrounding the Ag core significantly improves the charge transfer dynamics and overall energy storage capacity of the electrode. This mechanism revealed improved specific capacitance and rate capability of the NCA₁₅-MOF/NF electrode. The combination of enhanced surface area and improved charge transfer pathways due to Ag incorporation leads to better electrochemical performance, as demonstrated in the GCD and CV results.

Fig. 7 Proposed mechanism showing Ag particles as nucleation sites for MOF growth, increasing surface area and enhancing charge transfer pathways for improved electrochemical performance for SC applications.

3.3 Assembly and electrochemical performance of flexible symmetric SCs

Flexible quasi-solid-state symmetric SCs were successfully fabricated using NCA₁₅-MOF/NF as both the positive and negative electrodes. As shown in Fig. S9a and b (ESI†), the device demonstrated excellent flexibility at angles of 0°, 45°, and 90°, with a thickness of approximately 2 mm. The electrochemical performance of the SCs was significantly improved, exhibiting excellent pseudocapacitive properties. The structural arrangement of the SC assembly is shown in Fig. S9c and S10 (ESI†). A gel electrolyte was utilized in this configuration to achieve the desired characteristics, consisting of a PVA matrix cross-linked with solvent molecules and KOH serving as the electrolyte salt. The fabricated devices were subjected to CV measurements using scan rates ranging from 10 to 100 mV s⁻¹ at a potential of 1.6 V. We selected a 1.6 V potential window for the device to avoid increased polarization and a higher proportion of water electrolysis reactions that would happen with a wider voltage range. This choice was made to prevent adverse effects on the energy storage performance (Fig. 8a). The CV curves obtained at different scan rates (10 mV s⁻¹, 20 mV s⁻¹, 30 mV s⁻¹, 50 mV s⁻¹, and 100 mV s⁻¹) demonstrated the electrochemical behavior of the device (Fig. 8a). The quasi-rectangular shape of the CV curves, particularly at lower scan rates, suggested excellent capacitive behavior. This implies efficient charge storage due to the electric double-layer and faradaic reactions associated with the active material (Fig. 8a).

Fig. 8 Electrochemical characterization of the assembled SC device: (a) CV curves at different scan rates, (b) GCD profiles at different current densities, (c) Nyquist plots from EIS data, (d) cyclic specific capacitance and Coulomb efficiency, and (e) comparison of Ragone plots of the assembled SCs with previously reported MOFs.

The GCD curves at various current densities (0.5 A g⁻¹, 1 A g⁻¹, 2 A g⁻¹, and 4 A g⁻¹) provided insights into the charge–discharge characteristics of the device (Fig. 8b). The quasi-triangular shape of the GCD curves indicated good capacitive behavior, where the charge and discharge profiles are linear, further confirming the SC capacitive storage mechanism. Based on the GCD results and using eqn (S7) and (S8) (ESI†), it was found that the electrode achieved its highest energy density of 72.55 W h kg⁻¹ at a power density of 408.61 W kg⁻¹, corresponding to a current density of 0.5 A g⁻¹. Furthermore, the capacitances of the NCA₁₅-MOF/NF electrode were measured to be 215.24, 190.45, 181.87, 159.00 and 136.8 F g⁻¹ at current densities of 0.5 A g⁻¹, 1 A g⁻¹, 2 A g⁻¹, 4 A g⁻¹ and 5 A g⁻¹, respectively, with corresponding coulombic efficiencies of 98.34%, 83.08%, 85.42%, 90.80% and 90.34% (Fig. 8b). These values revealed the capacity of the device to store charge efficiently at lower current densities, while the drop in specific capacitance with increasing current density indicated a decrease in charge storage due to reduced ion diffusion at higher rates. Similarly, the energy density of the device was found to decrease with increasing current density. At 4 A g⁻¹, the lowest energy density of 14.04 W h kg⁻¹ was recorded, along with a power density of 3198.99 W kg⁻¹ (Fig. 8b). These results further confirmed the trade-off between energy and power densities at varying current densities, which is typical for SCs.

To evaluate the electrochemical processes occurring at the electrodes, EIS measurements were performed in the frequency range of 10⁻²–10⁴ Hz, as depicted in Fig. 8c. The results demonstrated the efficient operation of the device at a voltage of 1.6 V, indicating its excellent electrochemical performance. The durability of the device was assessed through 8000 charge–discharge cycles (Fig. 8e). The device retained approximately 85.9% of its initial specific capacitance, demonstrating its long-term characteristics. Moreover, the device maintained a consistently high Coulombic efficiency, ranging from 97% to 100%, thanks to its stable structure. However, the capacitance decreased after cycling (Fig. 8d). This decline might be due to changes in the material impedance, while the values of R₁ and R₂ remained relatively constant. Similarly, the W_O value decreased from 3.88 Ω to 2.32 Ω, resulting in an increase in the ion diffusion resistance and a further decrease in capacitance value. In addition, a Ragone plot was generated to compare the performance of the symmetric SCs used in this study with those reported in previous studies on MOF materials (Fig. 8e).^55–61 Additional details of the reported materials are provided in Table S3 (ESI†).

The device under investigation was subjected to CV and GCD measurements at various potential ranges to determine the optimal operating range (Fig. 9a and b). It was found that as the potential range expanded, the electrochemical performance of the device continuously improved without compromising consistency. However, a significant increase in the electrode material polarization was also observed, as is evident in the upper right corner area in Fig. 9a and b. This polarization was caused by both ohmic polarization, which occurs when current flows upon applying a potential, and concentration polarization, which is induced by changes in the ion concentration within the electrolyte. A working potential range of 0–1.6 V was preferred for the device to prevent aggravation of electrode polarization and maintain the electrochemical performance. Furthermore, the performance of the device was assessed under different bending angles using CV and GCD curves at bending angles of 0°, 45°, and 90° within a working range of 0–1.6 V (Fig. 9c and d). Impressively, the device demonstrated stable electrochemical performance even under bending at various angles, indicating its excellent flexibility.

Fig. 9 Characterization of SC devices: (a) CV, (b) GCD at different potentials, (c) CV, and (d) GCD at different bending angles.

4. Conclusion

This study developed a one-step solvothermal method for the self-growing synthesis of bimetallic (NiCo) MOF materials on flexible 3D-NF substrates. The incorporation of Ag into the NCA₁₅-MOF/NF electrode matrix significantly enhanced the specific capacitance to 1317 F g⁻¹ at 1 A g⁻¹ compared to 422 F g⁻¹ at 1 A g⁻¹ for the NCA₀-MOF/NF electrode without Ag. The incorporation of Ag resulted in the formation of uniformly sized spherical NiCo-MOF particles with a high density of interconnected pores, which contributed to the enhanced electrochemical performance. Moreover, using NCA₁₅-MOF/NF as both positive and negative electrodes, flexible quasi-solid symmetric SCs were assembled, exhibiting favorable capacitance and practical performance within a working potential window of 0–1.6 V. The SCs demonstrated excellent stability, retaining 85.9% of their initial specific capacitance even after 8000 charge–discharge cycles. Furthermore, the device exhibited consistent electrochemical performance and mechanical flexibility when subjected to various bending stresses at 0°, 45°, and 90°, as evidenced by the CV and GCD profiles. This study demonstrated the influence of large-radius metal ions on the electrochemical application of MOFs in supercapacitors.

Data availability

The data that support the findings of this study are available from the corresponding author upon reasonable request.

Conflicts of interest

The authors declare that they have no conflicts of interest.

Acknowledgements

This study was supported by the Key R&D Program of Zhenjiang (GY2018016).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4tc02970d

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